Self-bonding curable silicone compositions are known to have a variety of uses, for example, as sealants, as caulking compositions, in the formation in situ of gaskets, and as potting compounds. In general, the most broadly applicable such compositions are the RTV's, (room temperature vulcanizables), particularly those which cure under the influence of atmospheric moisture. Self-bonding compositions of the RTV type are described, for example, in Smith and Hamilton, U.S. Pat. No. 3,689,454. A particularly advantageous low-modulus self-bonding composition is described in the copending application of Beers, Ser. No. 579,025, filed May 19, 1975, and assigned to the assignee of the present application. The disclosures of the patent and of the said copending application are incorporated herein by reference to save unnecessary detail.
One problem commonly faced with many such compositions is their relatively low ability to bond to a wide variety of substrates, such as aluminum, stainless steel, brass, polycarbonate, polyacrylate, polyolefin, polyvinyl chloride, cardboard, glass, and the like. Especially significant substrates for bonding purposes with compositions of the RTV type are stainless steel and polyacrylate.
It has been proposed to use isocyanurates as adhesion promoters for such compositions. As an example, in the compositions of the RTV type described by Smith and Hamilton, the addition of tris-1,3-5-trimethoxysilylpropylisocyanurate or bis-1,3-trimethoxylsilylpropylisocyanurate, and analogs thereof has been found to promote adhesion to a variety of substrates. See Berger, U.S. Pat. Nos. 3,494,951; 3,498,852, and 3,821,218, each of which is incorporated herein by reference.
It has now been discovered, as part of an extensive program to improve adhesion in self-bonding silicone compositions of the RTV type, that a family of carbamates or derivatives thereof, known as allophanates, exists which has substantial superiority as adhesion promoters to compounds known to date. When compositions comprising predominantly such compounds are included in self-bonding silicone compositions of the RTV type described above, before curing, the final products adhere in a superior fashion to all common substrates, including the polyacrylates and stainless steels.